Immersion agent, coupling device and method for coupling an optical waveguide

ABSTRACT

The invention is directed to an immersion agent that can be used to couple optical waveguides to optical components. The invention is also directed to the use of an immersion agent, a coupling arrangement and a method for coupling optical waveguides to other optical components; for example, an optical chip.

PRIORITY APPLICATION

This application claims the benefit of priority under 35 U.S.C. §119 ofGerman Patent Application No. 100 04 296.8, filed Feb. 2, 2000, and is anational stage filing under 35 U.S.C. §371 of PCT applicationPCT/DE01/00331, filed Jan. 26, 2001.

FIELD OF THE INVENTION

The invention relates to an immersion agent, a coupling arrangement anda method for coupling at least one optical waveguide (LWL) to an opticalcomponent (chip) or for connecting an optical waveguide to an opticalcomponent.

BACKGROUND OF THE INVENTION

An optical coupling arrangement is used to inject light or couple lightbetween two optical waveguide end faces; for example between the endface of an optical waveguide fiber composed of a core and cladding, andthe opposite end face of a waveguide structure present on a chip. Suchcoupling arrangements are used, for example, in optical filters thatoperate according to thc phased array principle. These have an injectionface in which light enters at a specific point, the output wavelength ofthe optical filter depending on the geometric position of the injectionpoint. Optical filters operating according to the phased array principleare used in particular as multiplexers or demultiplexers in the field ofoptical telecommunications transmission as they have low insertionattenuation and high crosstalk suppression.

German patent application DE 44 22 651.9 describes what is referred toas a phased array filter whose central wavelength is defined bypositioning the optical waveguide fiber which injects the light into thechip waveguide structure, and can thus be precisely aligned. This isdone by displacing the waveguide end faces in relation to one another.

It has already been proposed to change the position of the end face ofthe optical waveguide with respect to the injection face of the chip byvirtue of the fact that an element with a variable length is fitted withthe fiber and the latter is thus displaced in parallel with thedirection of expansion of the element with a variable length.

In order to implement optical coupling between an optical waveguide, anoptical waveguide fibre or fiber array and an optical element containingactive and/or passive elements, the optical waveguides must be held in adefined position with respect to the injection face of the chip andconnected to the corresponding waveguide structure. This is usuallycarried out by directly bonding the fiber ends to the chip. However, inthe above mentioned cases, direct bonding or welding of the fiber endsto the chip is not desired as this would change the necessary relativemovement between the fiber and the chip.

SUMMARY OF THE INVENTION

The invention is directed to an immersion agent or material that can beused to couple at least one optical wave to an optical component. Inaccordance with the invention, the immersion agent is a transparentelastomer material having, among other characteristics, a modulus ofelasticity less than 200 N/cm². In the preferred embodiments of theinvention, the immersion agent is a two part material that can be mixed,in selected ratios to attain a selected degree of hardness, prior to usein coupling an optical waveguide to an optical component.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 illustrates a coupling arrangement between an optical fiber andan optical component using an immersion agent.

DETAILED DESCRIPTION OF THE INVENTION

In accordance with the present invention, in order to improve theinsertion attenuation and reduce the power fluctuations, an immersionagent has been introduced between the fiber and the chip. In particulara stable gel, for example, an additional crosslinked silicone rubber isused as the immersion agent. The silicone rubber is composed here of twocomponents whose mixing ratio is 1:1 so that the rubber hardenscompletely after its introduction. The hardening is considered necessaryin order to prevent the immersion agent flowing away from the joininglocation of the optical waveguide and the optical component. Inalternate embodiments as described below other mixing ratios can be usedto obtain a different degree of hardness of the cured immersion agent.

As an enclosed volume is formed between the fiber and its fiber mount onthe one hand and the chip on the other, fractures or vacuoles (smallvacuum bubbles) which considerably increase the insertion attenuation ofthe component are formed in the immersion gel during cooling as a resultof shrinkage. In addition, the formation of fractures is also promotedby the above mentioned relative movement between the fiber and chip.

The invention is therefore based on the object of making available anoptical coupling between an optical waveguide, for example, a fiber or afiber array, and an optical component/chip that has a low insertionattenuation.

In order to achieve this object, an immersion agent for coupling opticalwaveguides to an optical chip is characterized in that a transparentelastomer serves as the immersion agent, the tensile strain at break ofthe elastomer being greater than 300% and its modulus of elasticityhaving a value which is smaller than 200 N/cm². The immersion agent istherefore adjusted to be so soft that in the case of cooling sufficientgel continues to flow out of the edge regions of the immersion andstresses which lead to fractures and vacuoles cannot build up. On theother hand, the immersion material is not so liquid that it can flow outof the volume between the optical waveguide and chip, thus ensuring thatthe immersion material remains in this intermediate space during theentire service life of the component.

One advantageous configuration of the immersion agent according to theinvention is c that the matrix of the immersion agent contains anon-crosslinked proportion of a liquid phase. That is, the immersionagent is not completely crosslinked. By correspondingly adjusting themixing ratio, for example of a twocomponent immersion agent, it is thuspossible to adjust the immersion agent so that it is soft in the desiredway.

A further advantageous configuration of the immersion agent according tothe invention is c that the immersion agent contains a liquid phaseproportion of 1 to 10% of an immersion liquid with a low vapor pressure.The addition of a liquid phase proportion is a simple alternative togenerating an immersion agent with the properties mentioned at thebeginning.

A further advantageous configuration of the immersion agent according tothe invention is that an immersion oil or a softener which is selectedas a function of the elastomer is used as the immersion liquid, theproperties of the immersion agent being easily adjustable by adding theimmersion oil.

A further advantageous configuration of the immersion agent according tothe invention is that the immersion agent is silicone rubber, and inthat the immersion liquid is a silicone softener, in particular siliconeoils. As the silicone rubber has a pronounced degree of adhesivenesseven in the state in which it is not completely crosslinked, stabilityof the coupling of the component is ensured over its entire servicelife.

A further advantageous configuration of the immersion agent according tothe invention is that the immersion agent is epoxy acrylate, and in thatthe immersion liquid is an epoxy acrylate softener, in particularpolyisobutylene.

A further advantageous configuration of the immersion agent according tothe invention is that the immersion agent is urethane acrylate, and inthat the immersion liquid is a urethane acrylate softener, in particularpolyisobutylene.

The two last-mentioned immersion agents are also referred to asradiation crosslinked elastomers that constitute an advantageousalternative to silicone rubber. These immersion agents can also have theproperties required of the immersion agent according to the invention.

A further advantageous configuration of the immersion agent according tothe invention is that the immersion liquid or the softener is analiphatic or aromatic oil so that a series of softeners is available, itbeing possible to select the specific softener according to therespective peripheral conditions.

A further advantageous refinement of the immersion agent according tothe invention is that the silicone rubber is composed of two components,the mixing ratio of the components being 0.5:1 to 0.9:1. In the case ofsilicone rubber, for example in the case of the silicone rubber WACKERSilGel® 612 from Wacker-Chemie GmbH, a mixing ratio of the components of1:1 is recommended in order to obtain complete crosslinking of thesilicone rubber. If this mixing ratio is changed in the way stated, asilicone rubber is obtained which has the softness required according tothe invention.

A further advantageous refinement of the immersion agent according tothe invention is that the immersion agent has an adhesive force per unitarea which is greater than half the material breaking stress of theelastomer, it being advantageously ensured that the immersion agent doesnot become detached from the coupling face of the fiber or of the chipwhen a shrinkage process occurs.

A further advantageous refinement of the immersion agent according tothe invention is that the transformation point/glass transition point ofthe immersion agent T_(g) is below 0° C. This advantageously ensuresthat the immersion agent has the desired elastic properties over theentire operating temperature range of the coupling.

A further advantageous refinement of the immersion agent according tothe invention is that the refractive index of the immersion agent has avalue between n=1.3 and n=1.7, in particular between n=1.4 and n=1.5. Bythus selecting the refractive index, the coupling arrangement is adaptedin an optimum way to the refractive indices of the fibers or of thechip, and the coupling attenuation is correspondingly reduced.

In order to achieve the above mentioned object, an arrangement forcoupling optical waveguides, for example optical fibers or a fiberarray, to an optical component/chip is characterized in that

-   -   (a) an immersion agent of the above mentioned type is used, in        that    -   (b) the distance between an end face of the optical waveguide        and a coupling face of the chip is 2 μm to 20 μm, and in that    -   (c) the volume of the immersion agent applied to the coupling        point is less than 5 μl.

In trials it has been shown that both the properties of the immersionagent and the spatial conditions in the region of the couplingarrangement, in particular the distance between the optical waveguidesand the coupling face of the chip and the volume of immersion agent,influences the quality of the coupling and the service life of thecoupling arrangement. The formation of fractures or vacuoles canadvantageously be reduced further by this arrangement if the distancebetween a coupling face of the optical waveguide and the coupling faceof the chip is between 2 μm and 20 μm, and if the volume of theimmersion agent applied to the coupling point is below 5 its. If thedistance between the coupling faces is less than 2 μm, there is the riskof the coupling faces touching one another or of the friction betweenthe coupling faces being so large that the immersion agent becomesdetached from the coupling faces. If the distance is greater than 20 μm,the attenuation increases considerably as a result of the immersionagent. On the other hand, compliance with these parameters ensures thatif a shrinkage process occurs sufficient immersion agent can flow fromthe outside into the space between the coupling faces so that theformation of vacuoles is avoided.

A further advantageous refinement of the arrangement according to theinvention is one in which the optical waveguide fiber is arranged forcoupling in a ferrule, and is characterized in that the fiber is held ina ferrule, that the ferrule has a reduced diameter at its end side, andthat the coupling face of the chip is also reduced. This ensures thatthe distance from the edge region of the volume between the two couplingfaces and the center of this region becomes smaller so that if ashrinkage process occurs the gel can better and more quickly compensatestresses which possibly occur in order to avoid fractures and theformation of vacuoles.

A further advantageous refinement of the arrangement according to theinvention is that the coupling device is surrounded by a sealingcompound which has the same components as the immersion material but isadjusted so as to be harder, and is in particular completelycrosslinked. By filling the coupling point with casting material,mechanical shock stresses and vibrations are then only transmitted tothe coupling arrangement in an attenuated fashion. If the sealingcompound is made of the same immersion material as the immersionmaterial for the coupling, for example, also of silicone rubber,chemical compatibility between the two materials is advantageouslyobtained, which can differ only in the degree of hardening or in anaddition of softener in the case of the immersion agent at the couplingpoint. In addition, the coefficients of expansion of the two materialsare essentially identical so that when there are expansion or shrinkageprocesses no additional compressive forces or tensile forces are exertedon the immersion material at the coupling point. Finally, the elastomerimmersion material is advantageously protected against flowing away bythe sealing compound.

In order to achieve the above mentioned object, a method for couplingoptical waveguides, for example optical fibers or a fiber array, to anoptical chip is characterized in that

-   -   (a) an optical waveguide or a ferrule connected thereto is moved        to a distance of 2 to 20 μm away from a coupling face of the        chip, in that    -   (b) an immersion material of the above mentioned type is        prepared, in that    -   (c) the immersion material is dispensed at the coupling point to        a quantity of approximately 5 μl, and in that    -   (d) the immersion material is allowed to harden in accordance        with the mixing ratio.

Maintaining the spatial relationship between the coupling face of theoptical waveguide and the coupling face of the chip, that is to sayselecting the distance between these coupling faces, ensures, without alarge degree of expenditure during the coupling method, that during ashrinkage process or expansion process, a considerable formation offractures or vacuoles occurs even while the coupling arrangement isoperating. The coupling method itself is as simple here as in the priorart so that no additional expenditure is necessary to implement theinvention. Maintaining a relatively large distance between the couplingfaces also contradicts the previous practice according to which it wasattempted to arrange the two coupling faces as close to one another aspossible in order to improve the coupling of the light beam from theoptical waveguide into the chip. However, this coupling is surprisinglyworse if the difference between the coupling faces is too low becauseother mechanical and stress influences then act to the effect that theimmersion material can no longer bring about adequate optical couplingbetween the fiber and the chip.

A further advantageous refinement of the method according to theinvention is that, after the hardening of the immersion material, asealing compound which has the same components as the immersion materialbut is adjusted so as to be harder, and is in particular completelycrosslinked, is cast around the coupling point. As the same startingmaterial is used for sealing the component and also as an immersionmaterial for the coupling, this material merely has to be adjustable todifferent hardnesses in order to be suitable for this purpose. By usingthe same starting material, not only the above mentioned advantages areobtained but also stockholding is improved as it is not necessary tokeep different materials for the two purposes of use in stock.

A further advantageous refinement of the method according to theinvention is that the immersion material or the sealing compound isallowed to harden at normal operating temperature. This advantageouslyavoids the situation in which the material hardens at a temperaturewhich differs considerably from the normal operating temperature so thatthe component is already subjected to stresses when it is moved from theproduction site to the location where it is to be used. These stressesthen occur in addition to the “normal” stress loading that occurs duringthe operation of the component. If the original hardening of thecomponent has taken place at the normal operating temperature, theformation of fractures and vacuoles will also be less in the cases atthe operating site if the component has in the meantime been located inenvironments with different temperatures.

A further advantageous refinement of the method according to theinvention is c that the immersion material whose matrix contains aproportion of a liquid phase is manufactured in that a completelyhardened immersion material which is applied to the coupling point istreated with a softener. As a result of this method, already existingcoupling arrangements can advantageously be protected against futurefractures and the formation of vacuoles. In other words, by applying asuitable oil or another softener liquid to a hardened immersion materialthe immersion material is placed in a state which is suitable for thepurposes according to the invention.

A further advantageous refinement of the method according to theinvention is that aliphatic or aromatic oils are used as immersion oils.These oils are also suitable for the subsequent conditioning of theimmersion material for the purposes of the invention so that the desiredpurpose can be achieved without relatively large additional expenditure.

Exemplary embodiments of the invention will now be described withreference to the appended drawing in which a coupling arrangement forcoupling an optical waveguide fiber to an optical component/chip isillustrated schematically.

FIG. 1 illustrates a coupling arrangement between an optical fiber 2which is anchored in a ferrule 4 and an integrated optical component,for example, a chip 6 with waveguides (not shown) to be coupled. Animmersion agent or immersion gel 10 whose volume of approximately 5 μlor less fills the intermediate space between the end side of the fiber 2or the ferrule 4 and the coupling face on the chip 6 as well as an edgeregion surrounding this coupling region is provided at the couplingpoint 8. The parts of the coupling arrangement are sealed by a sealingcompound 12 in a housing 14.

The immersion agent 10 has the following properties:

-   -   (a) It has a refractive index between 1.3 and 1.7; preferably        between 1.4 and 1.5.

(b) It is a transparent elastomer with a tensile strain at break of over300%. The tensile strain at break can easily have values of 1000%. Giventhe coupling arrangements mentioned at the beginning, which permit amovement between the fiber and the chip, lateral displacements betweenthe two coupling faces of 30 to 40 μm occur. Given a distance betweenthe two coupling faces of 3 μm, a tensile strain at break ofapproximately 1000% occurs.

(c) The immersion material has a modulus of elasticity below 200 N/cm²,preferably below 100 N/cm².

(d) The immersion material has a transformation point/glass transitionpoint T_(g) below 0° C.

The immersion material has the required softness or adhesiveness byvirtue of the fact that, for example in the case of silicone rubber, themixing ratio of the two components of 0.5:1 to 0.9:1 is selected.Alternatively, a liquid phase proportion of softener or immersion oil toa quantity of 0% to 10% of the quantity of the immersion material can beadded, the immersion liquid having a low vapor pressure.

The immersion agent has an adhesive force per unit area in comparisonwith quartz glass of at least half the material breaking stress of theelastomer.

The sheet thickness of the immersion material between the coupling facesis between 20 μm and 20 μm, while the overall length of the immersionmaterial per fiber coupling is below 5 μm.

The material of the sealing compound is the same starting material asthe immersion material, but the sealing compound is completely hardened.The sealing compound advantageously protects the elastomer immersionmaterial against flowing away.

1. An immersion agent for coupling at least one optical waveguide to anoptical component, the immersion agent comprising a transparentelastomer, wherein the tensile strain at break of the elastomer isgreater than 300% and its modules of elasticity is smaller than 200N/cm².
 2. The immersion agent according to claim 1, wherein the matrixof the immersion agent further comprises a non-crosslinked proportion ofan immersion liquid phase.
 3. The immersion agent according to claim 2,wherein the immersion agent is silicone rubber and the immersion liquidis a silicone softener.
 4. The immersion agent according to claim 3,wherein the immersion liquid is a silicone oil.
 5. The immersion agentaccording to claim 3, wherein the silicone rubber is mixed from twocomponents, the mixing ratio of the components being in the range of0.5.1 to 0.9.1.
 6. The immersion agent according to in claim 2, whereinthe immersion agent is urethane acrylate and the immersion liquid is aurethane acrylate softener.
 7. The immersion agent according to claim 6,wherein said liquid is polyisobutylene.
 8. The immersion agent accordingto claim 1, wherein the immersion liquid is immersion oil or a softenerwhich is selected as a function of the elastomer.
 9. The immersion agentaccording to claim 8, wherein the softener is an aliphatic or aromaticoil.
 10. The immersion agent according to claim 7, wherein the immersionagent is epoxy acrylate, and the immersion liquid is an epoxy acrylatesoftener.
 11. The immersion liquid according to claim 10, wherein saidliquid is polyisobutylene.
 12. The immersion agent according to claim 1,wherein the immersion agent has an adhesive force per unit area which incomparison with glass is greater than half the material breaking stressof the elastomer.
 13. The immersion agent according to claim 1, whereinthe transformation point/glass transition point of the immersion agentT_(g) is less than 0° C.
 14. The immersion agent according to claim 1,wherein the refractive index of the immersion agent is in the range ofapproximately 1.3 to 1.7.
 15. The immersion agent according to claim 14,wherein the refractive index of the immersion agent is in the range ofapproximately 1.4 to 1.5.
 16. An arrangement for coupling at least oneoptical waveguide to an optical component, said arrangement comprising(a) and immersion agent comprising a transparent elastomer, wherein thetensile strain at break of the elastomer is greater than 300%, and itsmodulus of elasticity is smaller than 200 N/cm² (b) the distance (d)between an end face of the optical waveguide and a coupling face of thecomponent is in the approximate range of 2 μm to 20 μm, and (c) thevolume of the immersion agent applied to the coupling point (8) is lessthan 5 μl.
 17. The arrangement according to claim 16, wherein theoptical waveguide fiber being arranged for coupling in a ferrule, saidferrule having a reduced diameter at its end side and the coupling faceof the component is also being reduced to accommodate said ferrule. 18.The arrangement as claimed in claim 16, wherein the coupling device issurrounded by a two component sealing compound which has the samecomponents as the immersion agent, but wherein is adjusted so as to beharder by increasing the amount of hardening agent used in the sealingcompound.
 19. The arrangement as claimed in claim 18, wherein theimmersion agent is completely cross-linked.
 20. A method for coupling atleast one optical waveguide to an optical component, said methodcomprising: (a) moving an optical waveguide and/or a ferrule connectedthereto to a distance of in the approximate range of 2 to 20 μm from acoupling face of the optical component; (b) preparing an immersion agentcomprising a two component mixed in a ratio in the range of 0.5:1 to0.9:1; (c) depositing immersion agent at the coupling point is an amountof approximately 5 μl; and (d) allowing the immersion agent to harden inaccordance with the mixing ratio.
 21. The method as claimed in claim 20,wherein after hardening of the immersion agent, a sealing compound whichhas the same components as the immersion agent but is adjusted so as tobe harder, is cast around the coupling point.
 22. The method accordingto claim 21, wherein the immersion material is completely cross-linked.23. The method according to claim 22, wherein the immersion material orthe sealing compound is allowed to harden at normal operatingtemperature.
 24. The method according to claim 23, wherein the immersionmaterial whose matrix contains a proportion of a liquid phase ismanufactured in that a completely hardened immersion material which isapplied to the coupling point is treated with a softener.
 25. The methodaccording to claim 24, wherein aliphatic or aromatic oils are used asimmersion oils.